Amplifying circuit arrangement with periodically varying load connected in the cathode circuit



Nov. 4, 1952 SALMET 2,616,987

AMPLIFYING CIRCUIT ARRANGEMENT WITH PERIODICALLY VARYING LOAD CONNECTED IN THE CATHODE CIRCUIT Filed March 26, 1949 AGf/VT Patented Nov. 4, 1952 OFFICE AMPLIFYING CIRCUIT ARRANGEMENT WITH PERIODICALLY VARYING LOAD CONNECTED IN THE GATHODE CIR- CUIT- Gaston Salmet, Courbevoie, France, assignor to Socicte Industrielle des Procedes Loth, Paris,

France Application March 26, 1949, Serial No. 83,574

' In France April 2, 1948 l I Claims. (G l. 179-171) This invention relates to power amplifiers for powers of, say, several times ten kilowatts and comprising an electron-discharge tube, preferablya screen-grid tube, having a cathode, a control-electrode, an anode and an auxiliary electrode. 7 I

In a particularly suitable form of a power amplifier of this type, the output voltage is taken from-animpedance which is included in the cathode lead and is entirely or partly common to the input and output circuits of the amplifying tube. Such power amplifiers are of particular importance for use in modulation amplifiers of transmitters of 50 or 100 kilowatts.

In the amplifying tube usedthe optimum load current is determined by the characteristic properties of the tube, for example the anode dissipation and the dissipation occurring at the auxiliary electrode.

By supplying, according to the invention, a voltage proportional to the cathode alternating voltage and'of greater amplitude to the auxiliary electrode, the peak load current can be materially increased without exceeding the permissible dissipation of the auxiliary electrode.

A voltage proportional to the cathode alternating voltage and of greater amplitude may be supplied to the auxiliary electrode in a simple manner, for example by including a transformer or an autotran'sformer coupled with the auxiliary electrode in the cathode circuit of the amplifying tube.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to theaccompanying drawing, given by way of example.

Fig. 1 represents a known cathode follower comprising a pentode.

Fig. 2 represents current-voltage diagrams for explaining the cathode follower shown in Fig. 1.

Fig. 3 represents a power amplifier according to the invention, and.

Fig. 4 shows current-voltage diagrams for explaining the power amplifier shown in Fig. 3;

In the circuit-arrangementshown in Fig. 1,

which comprises a pentode I connected as a cathode follower, the signals to be amplified are supplied, by way of input terminals 2, 3, to the control grid which is also connected to the negative terminal of a grid-voltage battery 4. The output voltage of pentode I is taken from a load resistance 5 which is included in the cathode lead and is common to the input and output circuits oi the amplifying tube, and of which the screen-grid voltage being designated Vgz.

end connected to the cathode is connected to the screen grid through a condenser 6. The anode of pentode I is connected directly to the positive terminal of an anode voltage battery I, the screen-grid being connected, across resistance 8, to the positive terminal of a screen-grid battery.

The operation of the circuit-arrangement represented will be explained more fully with reference to the current-voltage curves shown in Fig. 2. In the drawing, curves III, II, I2, I3 and I4 represent the cathode-current Ik in accordance with the cathode voltage Vk at different values of a voltage Va supplied to the control-grid, the The points of curves III-I4, at which the control grid voltage corresponds to the cathode voltage and at which grid current tends to flow, are interconnected by a dotted curve I5. 7

The tube is set at the working point A, characterized by a cathode current In, a cathode voltage Vk a control-grid voltage V m determined by the grid battery 4, and a screen grid voltage Var- In supplying, to the input terminals 2, 3, a signal to be transmitted, the voltage set up at the cathode resistance 5 having a value RI: is given by VR 'IIkRk, in other words the cathode voltage varies according to a straight line I6 through theworking point A. If the tube is not controlled to a degree such that grid current occurs the maximum load current is determined by the point of intersection II of the straight line I 6 and the curve line I5.

In Fig.3 there is shown a cathode-follower in accordance with the invention, the circuit elements which correspond to those incorporated in the prior art device illustrated in Fig. 1 having identical reference numerals applied thereto. Thus the input signal is applied via terminals 2 and 3 to the grid circuit of pentode I, the amplified output signal being taken from across resistance 5 in the cathode circuit and applied to the load. As in the case of Fig. 1, the anode of pentode I is connected directly to the positive I? terminal of battery I, while the screen grid is connected via resistance 8 to the positive terminal of battery 9.

The circuit-arrangement shown in Fig. 3 is different from that represented in Fig. 1 in that the cathode-circuit of the former comprises an autotransformer I8, of which one tapping I9 is connected to the cathode and the upper end is connected, by way of a condenser 20, to the screen-grid. In this manner a voltage substantially proportional to the cathode alternating voltage and of larger amplitude is supplied to the screen-grid, which voltage is consequently in phase with the cathode alternating voltage.

In Fig. 4, curves 2 I, 22,, 23, 24 and 25 are cathode current/voltage characteristic curves at diiferent voltages set up at the control grid. Curve 26 shows the points at which the cathode-voltage corresponds to the voltage set up at the controlgrid.

If the tube l is not controlled to such a degree that grid-current occurs, the point of intersection 21 of load line 28 and curve 28 determines the maximum voltage set up at load resistance 5. At this cathode voltage the screen-grid voltage has a maximum value considerably exceeding that of the circuit-arrangement. shown in Fig. 1. Owing to this the maximum peak load current can be materially increased, as appears from Fi 4.

In the power amplifier according to the invention the product of screen-grid. voltage and screen-grid current for a maximum signal amplitude has a higher value than in the circuitarrangement shown in Fig. l, and for negative values of. the signal voltage the said product is much smaller. Screen-grid dissipation is ensured by determining the average values of. the product of the instantaneous screen-grid voltage and screen-grid current. If the amplifiers shown in Figs. 1 and 4 are set in. a manner such that equal screen grid dissipations occur, a higher peak-load current. may be taken from the power amplifier shown in Fig. 4.

What I claim is:

l. A power amplifier circuit. arrangement, comprising an electron discharge tube having cathode, control grid, screen grid and anode electrodes, an input circuit intercoupling said control grid and cathode electrodes, an output. circuit intercoupling said anode and cathode. electrodes, an impedance element coupled to said cathode-electrode and included in said input and output circuits, means to apply a. signal voltage to said input. circuit, means to derive an output voltage proportional to saidinput voltage from said impedance. element, means coupled to said impedance element to. derive, therefrom a control volt.- age proportional to and greater than said: output voltage, and means to. apply said control voltage to said screen grid electrode.

2. A power amplifier circuit arrangement, comprising an electron discharge tube having cathode, control grid, screen grid andjanode. electrodes, an input circuit intercoupling said control grid and cathode electrodes, an output circuit intercoupling said anode and cathods electrodes; an impedance element coupled to. said cathode electrode and included in said input and output circuits, means to apply a signal volt-age. to said input circuit, means to derive an output voltage proportional to said input voltage fromsaidimpedance element, transformer'means coupled to said impedance element to derive therefrom a control voltage proportional. to and greater than said output voltage, and. means: to apply. said control voltage to said screengrid electrode...

5 Number 3. A power amplifier circuit arrangement, comprising an electron discharge tube having cathode, control grid, screen grid and anode electrodes, an input circuit intercoupling said control grid and cathode electrodes, and output circuit intercoupling said anode and cathode electrodes, an impedance element coupled to said cathode electrode and included in said input and output circuits, means to apply a signal voltage to said input circuit, means to derive an output voltage proportional to said input voltage from said impedance element, an auto-transformer having a tapping, means to couple said tapping to said cathode electrode to produce across said autotransformer a control voltage proportional to and greater than said output voltage, and means to apply said control voltage to said screen grid electrode.

4. A power amplifier circuit arrangement, comprising an electron discharge tube having cathode, control grid-screen grid and anode electrodes, an impedance element intercoupling said cathode electrode and ground potential, means to apply a signal voltage between said control grid electrode and ground potential thereby to produce across said impedance element an output voltage proportional to said input voltage, an autotransformer having a tapping, means to couple one end of said auto-transformer to ground potential and the other end thereof to said screen grid electrode, and means to couple said tapping of said auto-transformer to said cathode electrode thereby to produce across said autotransformer a control voltage proportional to and greater than said output voltage and to apply said control voltage to said screen grid electrode.

5. A cathode-follower amplifier comprising an electron discharge tube having a cathode, a control grid, an auxiliary grid and an anode, means to apply an input signal to said control grid, an output impedance coupled to said cathode to provide an output signal which follows said input signal, means coupled to said impedance to derive therefrom a control voltage whose amplitude exceeds that of the output signal developed thereacross, and means to apply said control voltage to said auxiliary grid to raise the power amplification of said amplifier.

GASTON SALMET.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Robertson Feb. 27, 1940 Smethurst Feb. 27, 1940 Black Aug. 6, 1940 Blumlein Oct. 22, 1940 Veneklasen May 23, 1950 FOREIGN PATENTS Country Date Netherlands. Sept. 15, 1941 Number 

